This invention relates generally to materials and methods for manufacturing plastic parts and, more particularly, for manufacturing plastic shells or skins by applying plastic particles to a heated forming surface.
U.S. Pat. No. 5,998,030 recites that individual particles formed by the high intensity mixer process have a roughened surface with fissures and cracks therein. Further, the ""030 Patent goes on to recite that the use of such irregularly shaped particles in processing that includes feeding the particles to a point of use and casting the particles on a heated mold to melt flow the particles and cool them to form thin plastic shells presents several problems. In the case of slush molding, the particles are retained in a powder box that is rotated to direct an excess charge of material into a mold cavity. Such thermoplastic, irregular particles do not flow smoothly from all corners of a powder box. Furthermore, such particles do not smoothly flow into all parts of complex shaped molds of the type having tight return passages and very small mold surface features that simulate features such as leather grains, stitching or the like. As a result, it is necessary to vibrate the powder box and molds during the various processing steps so that the particles will flow against heated mold surfaces so as to melt and form a skin or shell shape corresponding to the shape of the heated mold surface. The irregular shape also produces an uneven build-up of material on the heated mold surface such that the particles do not melt and flow uniformly against the heated mold surface. As a consequence, the resultant cast part can have an irregular backside surface (i.e. opposite the mold surface). Such irregularities on the backside require that the nominal thickness of the part be larger than required for a given application which in turn takes more material than in the case of a part that has a uniform shape on its backside. The ""030 Patent goes on to disclose among other things, a thermoplastic microsphere comprising thermoplastic resin melt blended with pigment, said pigment comprising less than 5% by weight of said blend, said microspheres having an outer diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3.
According to one aspect of the invention, a plastic particle is provided comprising a plastic material and having an integral protruding ring section disposed on its outer surface. The ring may be continuous or non-continuous around a perimeter of the plastic particle. The particles optionally have an outer diameter in the range of 0.007 inches to 0.040 inches.
According to another aspect of the invention, a method of forming a plastic particle comprising a plastic material and having at least one integral protruding ring disposed on its outer surface is provided. The method comprises the steps of providing a plastic particle capable of forming at least one protruding ring on the outer surface of the plastic particle and heating the plastic particle until at least one integral protruding ring is disposed on the outer surface of the particle. The method may also include the step of cooling the plastic particle after the ring is disposed on the outer surface of the particle. The method may also include the steps of providing a die with at least one die hole, extruding the plastic material through the die hole, severing the extrudate to create a plastic particle, and cooling the plastic particle before the ring is disposed on the outer surface of the particle.
According to another aspect of the invention, a method of forming a plastic article from a plurality of plastic particles is provided. The method comprises the steps of providing a plurality of plastic particles, providing a forming surface for forming the plastic article, applying the plurality of plastic particles on the forming surface, heating the plastic particles while the plastic particles are disposed on the forming surface; and flowing the plastic particles while the plastic particles are disposed on the forming surface to form at least one integral protruding ring on the outer surface of at least one particle. The method may also include the steps of continuing to flow the plastic particles while the plastic particles are disposed in the forming surface to join the particles and form the plastic article, cooling the forming surface, and removing the molded article from the forming surface. Preferably, the plastic article comprises the shell or skin of an interior trim panel for a motor vehicle.